Part H says...
http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADH_2002.pdf Quote:1.17 Junctions on branch pipes of about the same diameter should be made with a sweep of 25mm or at 45 degrees. <snip>
1.18 Branch pipes upto 40mm diameter joining branch pipes 100mm diameter or greater should if practical connect to the upper part of the pipe wall of the larger branch
So I guess you could use a 45 degree branch and a reducer.
Rather than run a pipe in boxing along the bottom of a wall we choose to loose a little space and built a false wall. Same wall also has wall mounted taps and spout for a free standing bath.
The fall on the 110mm pipe needs to be around 1:40 or 60mm over the 2.2m.